An articulated main rotor assembly comprises a main rotor hub and a plurality of main rotor blades secured in combination therewith. An exemplary articulated main rotor assembly 100 is illustrated in FIG. 1 wherein reference numeral 102 identifies the main rotor hub and reference numeral 104 identifies a main rotor blade. Each main rotor blade 104 comprises an aerodynamic fairing 106 disposed in combination with an internal spar member 108. The internal spar member 108 functions as the primary load-carrying structural element of the main rotor blade 104, carrying most of the bending moments, twisting moments, shear, and centrifugal force induced in the main rotor blade 104.
Prior art spar members have typically been fabricated from titanium due to its high fatigue strength, torsional stiffness, and corrosion resistance. The trend in recent times, however, is to fabricate spar members as composite structures, e.g., a fibrous material such as graphite and/or fiberglass embedded in a resin matrix, due to the capability to fabricate a composite spar member having a torsional stiffness equivalent to its titanium counterpart, but at a significantly lower weight. In addition, a composite spar member is more advantageous in possessing almost twice the fatigue strain allowable of its titanium counterpart, as well as improved damage tolerance and attenuated crack propagation characteristics.
The spar member for an exemplary articulated main rotor assembly is an elongated structure that extends from the root to approximately the tip of the main rotor blade. The spar member has a closed tubular configuration for maximum torsional stiffness. The spar member functions as the attachment joint for securing the main rotor blade in combination with a cuff of the main rotor hub (the attachment joint and cuff are identified by reference numerals 110 and 112, respectively, in FIG. 1).
The cuff is typically bolted in combination with the spar member to secure the main rotor blade in combination with the main rotor hub. A problem encountered with this securement mechanism is that the compressive force generated by the torqueing of the bolts is coupled into the spar member. An excessive compressive force tends to bend or deform, i.e., collapse, a titanium spar member, which may degrade the load-carrying capability of the spar member. An excessive compressive force poses a more exacerbated problem in a main rotor assembly having composite spar members. An excessive compressive force may induce cracks into a composite spar member, degrading the load-carrying capability of the spar member and may lead to catastrophic failure thereof.
To counteract possible adverse effects due to the compressive force generated by attachment bolt torqueing, a solid one piece filler block 114, typically fabricated from a phenolic material for weight savings, is disposed in combination with the spar member 108. The filler block 114 is inserted in the spar cavity 116 at the root end 108R of the spar member 108 to abuttingly engage the inner mold line (IML) surfaces 108S of the spar member 108, as illustrated in FIG. 1A. The filler block 114 reacts the compressive forces generated by the bolt torqueing operation, thereby precluding compressive forces from being coupled into and deforming the spar member 108.
While a one-piece filler block is functionally adequate to protect the spar member from the adverse effects of compressive forces as a result of bolt torqueing, the forming of the filler block to net shape so that the inserted filler block abuttingly engages the IML surfaces of the spar member is both labor intensive and time consuming. Filler blocks are typically fabricated initially as rectangular blocks which are subsequently hand sanded to the required net shape. This typically involves an iterative process wherein the filler blocks are sanded, fitted, sanded, etc., until the required net shape is achieved. Such a procedure is per se labor intensive and time consuming. Another drawback with this procedure is that the filler block or segments thereof may be over sanded such that the filler block fits loosely in the spar cavity. Upon cuff bolt torqueing, the spar member may be slightly deformed, i.e., collapsed, due to the loose fit of the filler block. This could result in loss of bolt torque, undesirable changes in the shape of the airfoil fairing, or excessive stresses being induced into the spar member. The fitting of solid filler blocks is exacerbated for composite spar members. While the outer mold line (OML) of the composite spar member is tightly controlled, the IML surfaces of a composite spar member may have a rough, irregular surface texture, in addition to significantly large variations in spar inside dimensions due to composite material variations, e.g., resin content, fiber debulking, etc. In contrast, the inner surfaces of a titanium spar member have a smooth, flat texture. Rough, irregular IML surfaces pose a greater challenge in net shaping filler blocks, and increase the likelihood that the spar member may be subjected to deleterious effects during bolt torqueing.
A need exists to provide a filler device that reacts bolt torqueing to preclude induced deformation of the spar member. The filler device should be fittable within the spar cavity to abuttingly engage the inner surfaces of the spar member without the need for labor intensive and time consuming rework of the external configuration thereof. The filler device should be compatible for use in combination with the rough, irregular IML surfaces of a composite spar member as well as the smooth, flat inner surfaces of a titanium spar member.